Structure-activity studies show that successful inhibition of DNA topoisomerase I by camptothecin analogues requires an intact lactone ring (E-ring) functionality. Camptothecin analogs having open lactone ring structures (also known as the carboxylate form or as camptothecin carboxylates) are poorly accumulated by cancer cells, exhibit limited activity against the topoisomerase enzyme, and may be more toxic to healthy cells than the lactone form.
Unfortunately, in aqueous solutions, camptothecins undergo a pH dependent hydrolysis to the inactive carboxylate form (FIG. 1). This, coupled with the fact that the inactive carboxylate form is favored in aqueous solution at pH 7.4 may account for the limited clinical success of certain camptothecins. Even more, under physiological conditions such as in blood, this equilibrium may shift further toward the inactive carboxylate form by preferential carboxylate binding to serum albumin, which affects certain congeners more than others.
Camptothecins may be further subclassified into neutral and cationic camptothecins. Camptothecins which exist primarily as neutral species at pH less than 7 are generally classified as neutral, whereas camptothecins having cationic substituent groups in the parent structure and which exist predominantly as positively charged species at pH less than 7 are classified as cationic camptothecins. Both subspecies of camptothecins undergo chemical degradation by lactone ring hydrolysis to the inactive carboxylate form as described above. It is known that neutral camptothecins are significantly less water-soluble at low pH than their cationic counterparts.
This poor water-solubility has severely limited the types of formulations and drug concentrations of highly lipophilic neutral camptothecinsm, prodrugs and analogs [e.g., 10-hydroxy-7-ethyl camptothecin (SN38), silatecan 7-t-butyldimethylsilyl-10-hydroxycamptothecin (DB-67), karenitecan, gimatecan, irinotecan, 9-nitro camptothecin, and the like] that can be employed for clinical treatment of cancer, even though lipophilic camptothecins and analogs provide several important advantages over their water-soluble counterparts. For example, because of its increased lipophilicity and dual 7-alkylsilyl and 10-hydroxy substitution, DB-67 displays superior binding to cellular and liposomal membranes and enhanced drug stability in the presence of human serum albumin when compared with clinically relevant, more hydrophilic camptothecin analogues. Indeed, in vitro cytotoxicity assays indicate that DB-67 has at least comparable potency with other FDA approved camptothecin analogs (e.g., Camptosar and Hycamtin).
It is known to utilize liposomes as delivery vehicles for camptothecins. Liposomal encapsulation can improve solubility of both neutral and cationic camptothecins, overcoming known water insolubility of camptothecins and analogs and also minimizing side effects of camptothecins relating to their cytotoxicity by more closely targeting delivery to tumor tissue. In addition, it has been shown that liposomes may accumulate in tumor tissue after delivery due to an enhanced permeation and retention effect ((Drummond, D. C., Meyer, O., Hong, K., Kirpotin, D. B., Papahadjopoulos, D. 1999, Optimizing liposomes for delivery of chemotherapeutic agents to solid tumors, Pharmacol. Rev. 51(4): 691-743)), providing a more targeted drug delivery and enabling passive tumor targeting. That is, drug encapsulation in liposomes can improve drug distribution (preferentially to tumor tissue rather than normal tissue) by reducing drug access to normal tissue. Specifically, the enhanced permeability of the microvasculature in tumor tissue allows particles having the size range of a liposome to escape from the blood circulature and collect in tumor tissue. Once in the tumor bed, particles such as liposomes are retained in tumor tissue for an increased period of time.
Camptothecins in the active lactone form partition preferentially into liposomal membranes, thus minimizing exposure to the aqueous environment and decreasing unwanted conversion of the active lactone form to the inactive carboxylate form. For this reason, typically a low intraliposomal pH is maintained to stabilize camptothecins held therein in the active lactone form. This formulation strategy typically requires use of an aqueous buffer and, while effective for cationic camptothecins, is unsuitable for neutral camptothecins due to their reduced water-solubility at low pH as noted above. Accordingly, while liposomal delivery technology is an effective method of delivery for camptothecins, presently it is less suited to delivery of neutral camptothecins due to the difficulties in achieving adequate concentrations of liposome-bound drug, and also to inadequate retention of the lactone form of the drug in liposomes after injection. Indeed, inadequate retention of lactone-form neutral camptothecins in liposomes after delivery to patients limits or eliminates any advantage provided by liposomal delivery compared to conventional dosage forms such as injectable solutions.
Especially for the advantages provided by use of neutral camptothecins compared to cationic camptothecins, that is, enhanced membrane-binding and superior drug stability in the presence of blood components such as albumin, there remains a need in the art for pharmaceutical formulations comprising neutral camptothecins for treatment of various cancers. Such formulations should provide a stable camptothecin formulation, and also improve retention of intraliposomal drug, promoting passive targeting of tumor tissue by liposomes and delivery of increased dosages of drug to such tumor tissue while reducing effects on healthy tissue.